Traditionally, images, movies, slide shows, and the like, are projected along a horizontal plane against a wall or screen. Less common alternatives to horizontal projection include vertical and/or ceiling projection as part of an immersive environment, for instance. Projection onto a ceiling surface includes dome type surfaces. A dome is a structural element of architecture that resembles the hollow upper half of a sphere. Domes do not necessarily have a cross-section of a sphere, and some domes may be formed by using an ellipse. A dome can be formed by taking an arch and rotating the arch around its vertical axis.
Tall domes and low domes are possible. A very low dome is a saucer dome. Another type of dome is an oval dome. A saucer dome is a low pitched shallow dome that has a circular base and a segmental (less than a semicircle) section. A section across the longer axis results in a low dome, capping the volume. Many of the largest existing domes are saucer domes. The onion dome resembles more than half of a sphere and is often found in eastern architecture.
Typically, all the surfaces of a dome are curved. In architectural construction, vaulted spaces are often formed enclosed beneath a concrete dome structure. A small dome can be constructed of ordinary masonry, held together by friction and compressive forces. A dome can sit directly on a circular base, or on another base shape such as a square. The concave triangular or trapezoidal sections of vaulting that provide the transition between a dome and the square base on which it is set are called pendentives, which transfer the weight of the dome to the base. The pendentives, which are triangular segments of a sphere, taper to points at the bottom and spread at the top to establish the continuous circular or elliptical base needed for the dome. In a simple dome, the pendentives are part of the same sphere as the dome itself. Simple domes are rare. In the more common compound dome the pendentives are part of the surface of a sphere of larger radius than the dome itself but whose center is at a point lower than that of the dome. A less sophisticated version of a pendentive is a squinch. Domes have a great deal of structural strength and have several advantages for projection.